The long-term objective of this research program is to study basic structural and functional aspects of plasma and platelet factor XIII and to relate these to the enzymatic activity of factor XIIIa in physiologic and pathologic processes. Factor XIII is essential for normal hemostasis; and it may be important in certain pathological processes such as thrombosis, wound healing, and tumor growth and metastasis. The factor XIIIa catalyzed crosslinking of fibrin to fibronectin and of certain platelet proteins may be important in some of these processes. There are five specific objectives in this research proposal. (1) We will use our specific radioimmunoassays for the a and b subunits of factor XIII to study subunit interactions, especially dissociation of the tetramer during activation of the plasma zymogen and recombination of purified a and b subunits. (2) We will study the interactions of factor XIII with its substrates, particularly fibrin, fibronectin, and fibrinogen and will determine binding affinities for the substrates. (3) We will carry out experiments on factor XIII activation and degradation, including studies on the effect of substrate on the susceptibility of factor XIIIa to plasmin degradation; studies on other enzymes which might have proteolytic activity toward factor XIII, such as platelet proteases and elastin; and studies on the direct activation of factor XIII in blood, using blood from a patient with congenital afibrinogenemia. (4) We will study metal ion interactions with factor XIII. These include nmr studies with 43Ca2 ion and other stable isotopes to study metal ion binding, including halide ion nmr to study the active site region, and Eu3 ion, EU2 ion ion luminescence lifetimes as another way to examine the active site. (5) We will combine our specific radioimmunoassays for the a and b subunits with activity measurements to assess factor XIII in groups of patients who may have abnormal function in association with other clinical disorders, such as thrombosis, DIC, liver disease, leukemias, and metastatic tumors. We will also continue our work on patients with congenital factor XIII deficiency.